The present invention relates to a fastener system including a threaded fastener having a determinable and visibly indicatable clamp load feature and in addition having an anti-loosening or prevailing torque type lock feature.
Threaded fastener assemblies are used in many applications for aerospace and industrial constructions. Such assemblies include threaded bolts or studs and nut members having a matching thread form.
In securing workpieces together with threaded fasteners, such as a nut and a bolt, it is desirable that the magnitude of final clamp load achieved be at a desirable magnitude which is readily verifiable. If the clamp load is too low, loosening of the joint could occur. If the clamp load is excessive this could result in damage to the workpiece and/or damage or excessive stress on the fastener. Frequently the magnitude of clamp load is determined substantially solely by measuring the torque applied between the nut and bolt. When the torque reaches a particular magnitude it is expected that a certain magnitude of clamp load has been attained. Unfortunately this is not uniformly predictable since the applied torque is resisted by the inherent friction between the engaged threads and other surfaces that are being rotated relative to each other. The magnitude of the friction can vary considerably depending upon numerous factors including the fit between the engaging threads, the lubricity, if any, between the engaged surfaces, temperature, installation environment and the like.
The threaded fastener construction of the present invention provides a unique way for predictably providing a selected, desired magnitude of final clamp load of the workpieces being joined. At the same time the fastener construction provides a means for readily verifying attainment of the desired magnitude of final clamp load. In addition the fastener has a unique construction for providing an anti-loosening or prevailing torque type lock feature between the nut and bolt without affecting the attainment and predictability of the desired magnitude of final clamp load.
In applications subject to vibration or repeated load fluctuations threaded fastener assemblies have been known to lose their retention torque and to loosen. To inhibit such action, threaded fastener assemblies have been modified to provide a prevailing torque feature to resist loosening and loss of clamp of the workpieces or articles which are joined. Frequently, a prevailing torque feature has been attained by providing one of the mating thread forms with a distorted portion such that, upon engagement between the distorted thread portion and the undistorted thread form, the nut member and the stud or bolt will be additionally held together under a prevailing torque by a thread interference which will resist untorqing and hence resist loosening from vibration.
With standard, non-distorted thread forms, the nut member is free running at low torque values onto the bolt or stud until workpiece engagement and initial clamp or preload on the workpieces. Thereafter the final clamp load attained relative to the magnitude of applied torque is resisted mainly by the inherent friction between the engaged portions, such as the threads, of the rotating members under load. This is not the case with threaded fasteners having a distorted thread form to provide a prevailing torque function . Here once the distorted thread portion is engaged by the mating threads the nut member is no longer free running and the clamp load of the workpieces being fastened relative to the magnitude of the applied torque is reduced. While the distortion of the thread form is often placed at the outer or trailing end of the nut, this still restricts the free running capability of the nut member onto the stud or bolt and the magnitude of initial preload and final clamp load relative to the applied torque. In addition the mating engagement between the distorted and undistorted threads may be somewhat grip sensitive over the grip range, i.e. with grip range being the variation in total thickness of workpieces, from a maximum grip or maximum total thickness to a minimum grip or minimum total thickness, and to be secured together by a single sized fastener. Thus in such systems the relationship between the magnitude of applied torque and the magnitude of final clamp load on the workpieces may be compromised
The present invention provides a unique nut and bolt assembly which not only provides a reliably predictable and verifiable magnitude of final clamp load but also provides an anti-loosening or prevailing torque type self-locking feature. Thus the fastener has a nut assembly which includes a threaded nut member with a separate non-threaded lock base secured or otherwise operatively connected thereto. The lock base has a load ring portion with an axially extending, collapsible boss by which it is operatively connected to the nut member and through which the clamp load is applied. As connected to the nut member the load ring portion is spaced from the nut member by a predetermined axial gap as determined by the length of the boss. As will be seen the boss is of a generally tubular construction. The nut assembly is substantially free running on an associated bolt or stud and permits clamping of the workpieces up to a first magnitude of torque. In this regard since the lock base is not threaded, the threaded action is between the nut member and the bolt or stud. The first magnitude of torque will provide a desired magnitude of initial clamp load or preload on the workpieces with the clamp load applied through column loading of the boss by the nut member. After this first magnitude of torque is attained in a substantially free running step, the axial force and column load applied to the boss increases in response to increased torque. At a second magnitude of torque the axial force is increased to a magnitude at which the boss collapses and is compressively deformed whereby the nut member moves towards the load ring portion of the lock base. The magnitude of axial force at which the boss collapses is determined by the known strength of the boss and hence will result in a determinable magnitude of clamp load. The torque and hence clamp load is continued to be increased deforming the boss radially into the threads of the mating bolt or stud with a high radial, compressive force. The torque is further increased until a third magnitude of torque is attained at which the gap between the nut member and the ring portion is substantially closed and at which point the installation is completed. The result is a fastened joint with the workpieces held together under a preselected magnitude of final clamp load. This final magnitude of clamp load is confirmed by the closure of the gap which occurs after the boss collapses at a predictable relative axial load applied between the nut assembly and bolt. It is this relative axial load which is in essence the clamp load. At the same time threads of the nut: member and the newly formed thread of the lock base engaged with the bolt or stud of the installed fastener are held together under a high radial, compressive load resulting in an anti-loosening, high prevailing torque type lock structure which provides resistance to removal and hence a desired resistance to loosening from vibration or other cyclic loading. In this regard it should be noted that, unlike conventional prevailing torque structures, here there is no thread distortion inhibiting free running of the nut and the attainment of a first, determinable magnitude of clamp load. The anti-loosening or prevailing torque type lock feature occurs upon collapse of the boss into compressive engagement with the bolt threads after the first magnitude of clamp load has been attained. It should also be noted that additional torque could be applied after gap closure to further increase the clamp load after gap closure. In this case the determination of the magnitude of final clamp load would be more readily predictable by measuring the amount of relative rotation applied between the nut assembly and bolt. In the fastener of the present invention, the connection between the nut member and lock base is such that rotation of the lock base by the nut member in installation is essentially precluded.
As will be seen this is a completely different concept from prior threaded fasteners having a lock nut having a neck connected to a washer by a frangible portion with the neck adapted to move into the washer bore to form a lock. See in this regard the U.S. Pat. No. 2,380,944 to Pummill for "SELF-LOCKING NUT OR BOLT" and U.S. Pat. No. 2,520,259 to Pummill for "SELF-LOCKING NUT AND WASHER". See also U.S. Pat. No. 1,464,591 to Weichold, Jr. for "LOCK NUT".
Thus in a preferred form of the present invention the nut assembly includes a nut member and a separate lock base with the clamp load being determined by a deformable or collapsible cylindrical boss on the lock base and with the radially compressive load applied to the threads of the bolt occurring after the boss has collapsibly deformed upon attainment of a determinable magnitude of clamp load.
The unique nut assembly of the present invention can be utilized with a variety of thread forms including generally standard thread forms available on existing bolts or studs.
The free running capability, determinable clamp load and resultant anti-loosening or prevailing torque type lock characteristic of the unique nut assembly are desirable for many industrial and aerospace applications and will be substantially uniformly operative over the grip range of the related fastener.
Thus it is an object of the present invention to provide a unique threaded fastener for securing workpieces together with a final clamp load which is determinable and visibly verifiable.
It is also an object of the present invention to provide a unique threaded fastener which, upon installation, has a desired anti-loosening or prevailing torque type lock characteristic.
It is another object of the present invention to provide a unique threaded fastener assembly for securing workpieces together and including a threaded bolt or stud and a free running nut assembly adapted to provide a preselected magnitude of initial preload on the workpieces without added interference or frictional loads between the engaging threads and to subsequently provide an anti-loosening or prevailing torque type lock characteristic upon attainment of a final, determinable clamp load.
It is still another object of the present invention to provide a unique threaded fastener assembly for securing workpieces together and including a threaded bolt or stud and a free running nut assembly adapted to provide a preselected magnitude of initial preload on the workpieces without added interference or frictional loads between the engaging threads and to subsequently provide a final clamp load on the workpieces which is determinable and visibly verifiable and also providing an anti-loosening or prevailing torque type lock characteristic upon attainment of the determinable final clamp load.
Other objects, features, and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, in which: